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DPS Meeting Day 1: Geysers on Enceladus

Another report from day one of the American Astronomical Society Division for …

Here is a second report from the conference's first day (October 11th), covering the session on Enceladus, an icy moon of Saturn. For this session, I will summarize multiple presentations that focused on determining what drives the geysers on Enceladus. The discovery of geysers near the south pole of the small, icy moon of Saturn has generated huge excitement in the planetary sciences community, and the Cassini mission has been dedicating a lot of its resources to study the phenomenon in detail.

First, for a geyser to be active, there must be sufficient heat to generate water vapor; thus, scientific instruments on board Cassini has been focusing on measuring the heat output of the geyser. In the region of geyser outgassing, Cassini has found four large stripes of crack-like features that have been named Alexandria, Baghdad, Damascus, and Cairo—the whole region has been called the Tiger Stripes.

Presentations by John Spencer and Carly Howett, both of Southwest Research Institute, used the Composite Infrared Spectrometer on Cassini to measure the warmest region of the Tiger Stripes, and found that it can get up to 200K, more than 100K warmer than the typical temperature observed on the rest of the satellite's surface. They also showed that the temperate measured around Baghdad has changed over the course of Cassini mission.

Near the end of the session, there were two talks that proposed models of how the heat, presumably generated deep inside the moon, gets transported to the surface. Andrew Ingersoll of California Institute of Technology (who happens to be my current boss, although I had no role in this study) discussed how we can get 6 GigaWatts of heat flux out of the geysers, a figure inferred from some observations. The phase of water (solid, liquid, or vapor) inside of the geyser region is still unknown, and Ingersoll is aiming to place constraints on that.

Since conduction through ice is an extremely slow process, he reasons that either vapor, liquid, or both are moving through cracks in the ice, and that this is responsible for transporting the heat from the deep interior to the surface. If we assume that vapor alone is transporting the heat, the vapor's pressure must be in equilibrium with the vapor pressure of surrounding ice. The pressure cannot be below the equilibrium since the ice would then sublimate and bring the pressure to the equilibrium level; if it's too high, the vapor would condense and freeze out to bring the system back to equilibrium. Thus, there is an upper limit on the pressure, if we assume that vapor is responsible for the heat transport.

Ingersoll calculates that this maximum pressure is insufficient to drive the geyser activities we observe unless the interior of Enceladus is extremely porous, which is highly unlikely. If we have a higher vapor pressures, water would condense and eventually melt ice, and liquid water will start contributing to the heat transport. He presented several other arguments as to why melting of ice is unavoidable in driving the geyser, and concludes that liquid water must play a significant role.

In the last talk of the session, Terry Hurford of NASA's Goddard Space Flight Center discussed the effects of tidal force exerted on Enceladus by the gravity of Saturn. The tidal force impacts the satellite in several ways. First, it leads to the heating of the interior, which is presumed to be the source of the heat that eventually erupts out of the vents in the Tiger Stripes region. The tidal force can also create cracks that help vapor and liquid water move toward the surface and transport the heat.

Finally, tidal flexing of the surface of the moon can open and close the cracks at the site of the vents, thus raising and lowering the internal pressure that blows the vapor out of the vents. Studying these effects in detail, Hurford estimates that the tidal heating in the interior must be at least 8 GW—this is the global heat budget of the moon, since no other heat-generating mechanism is known to be significant. Hurford predicts that the timing of the opening and closing of vents depends on how the shape of the moon flexes and vibrates in response to the tidal force, meaning that the timing of the eruptions may contain hints of the internal structures of the small icy moon.

To summarize, there is a synergetic collaboration going on between the observers and the modelers in figuring out why on Earth there are geysers on Enceladus. These studies are still ongoing, and I hope to write about them when the results start getting published, so stay tuned!

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